The invention relates to a process for treating water, in particular for obtaining ultrapure water. The invention further relates to a water treatment system and counter-current ion exchange unit for carrying out such a process.
The treatment of water is currently of constantly increasing importance, in particular in the field of obtaining drinking water, and also in the production of high-purity process water. Particularly high demands are made in respect of the water purity, in particular in the latter case, for example in the case of process waters which are required in the production of semiconductors. For instance, in semiconductor production, water having an extremely high degree of purity is required for washing silicon wafers, in particular after etching processes.
The starting point for producing the required high-purity water can be surface water, for example river water, or ground water (for example well water). This is typically purified in a multistage process comprising a pretreatment section, what is termed a “make-up” section, and what is termed a “polishing” section.
The pretreatment generally comprises in particular one or more filtration steps for removing rough and fine particles plus eventually a flocculation step for removing colloid substances and very fine dirt particles, an adsorption step (activated carbon usually), a softening step and/or a de-mineralisation step of the raw water by means of ion exchange resins and/or reverse osmosis membranes. Subsequently, the water thus treated is, in the make-up section, degassed, deionized and treated with UV. A further UV treatment can be provided during polishing. In addition, the polishing generally comprises further deionization processes and also at least one ultrafiltration step.
The raw water to be treated generally contains inorganic and organic constituents or impurities which must be removed as far as possible during the treatment. As regards such constituents and impurities, respectively, it is particularly referred to organically bound carbon (“total organic carbon, TOC”), silica and boron.
Boron can usually be removed as boric acid by means of strong base anion exchangers.
However, the affinity of these exchangers for boron is generally low resulting in a low removing capacity (mgram boron per liter strong base anion exchanger) and in a very early breakthrough (elution) of boron. Typically, boron breaks through as the first impurity when the strong base anion exchanger becomes exhausted.
In other words, when, for instance, breakthrough and elution, respectively of silica is detected at an outlet of a strong base anion exchanger, normally boron is already eluted to at least a significant extent.
Thus, for controlling boron removal, according to a conventional process, silica concentration is measured in a strong base anion exchanger, at approximately 20 to 30 cm upstream of the outlet of the exchanger, thereby relying on that short layer downstream of the silica measurement to catch the boron front displaced by the silica front.
However, this approach suffers from the withdrawal that no real boron control is possible (except through grab samples for laboratory measurements). Further this conventional process is only possible as long as the boron concentration in the feed water is stable and does not exceed a range of about 50 to about 100 ppb (parts per billions).